The experiments in this proposal are designed to investigate the frequency microstructure of both auditory thresholds and otoacoustic (OAE) phenomena. The clinical ramifications of frequency microstructure will be explored by examining ears with cochlear impairment. The ultimate goal of this line of research will be to develop more specific diagnostic OAE procedure. The specific questions of the proposal are: 1) Is the cochlear mechanism for frequency microstructure common to both auditory threshold and stimulus frequency OAEs (SFOAEs)? Direct comparison of the microstructure of both threshold and SFOAE from the same normal ears (in terms of frequencies of maxima/minima and amplitude excursion) will be used to evaluate and contrast several opposing hypotheses regarding underlying cochlear mechanisms. 2) Is frequency microstructure in distortion product otoacoustic emissions (DPOAE) frequency functions related to multiple generation sources? Comparison of DPOAE frequency functions as measured with several different primary stimulus protocols among each other and with auditory thresholds will be used to compare between two theories (Allen and Fahey, 1993; Stover, Neely and Gorga;, 1997) 3) Can empirical results be reproduced by, and the hypothesis implemented in a mathematical model of cochlear mechanics? Attempts will be made to incorporate random fluctuations in several aspects of the cochlear partition in a mathematical model of the cochlea (Neely, 1993) which will reproduce empirical data while allowing the model to remain stable. 4) Does loss of frequency microstructure occur in aging in the absence of hearing loss? Frequency microstructure across normal hearing and mild hearing loss ranges will be compared for two age groups of adults carefully matched for audiometric threshold. 5) Can DPOAE frequency microstructure account for the lack of specificity of DPOAEs in clinical diagnostics? Based on earlier experiments a shortened protocol of DPOAE frequency functions will be evaluated on a large population of subjects with normal hearing to mild hearing losses in an attempt to improve clinical estimation of auditory threshold.